The invention relates to an apparatus for imprinting a material web which is movable through the apparatus by a transport device and to which at least in a first station, a coloring agent is applied and, in a second station, an additional coloring agent is applied to the material web and/or the coloring agent is fixed to the material web.
Such an apparatus is conventional and is manufactured and offered for example by the company CSAT Gesellschaft für Computersysteme and Automatisierungs-Technik. With this known apparatus for example an aluminum foil which is taken from a foil roll can be color imprinted. To this end, the apparatus includes printing heads which are arranged in the transport direction of the foil one after the other, each applying a different coloring agent to the foil. In context with the present application, a black ink or a black toner is also considered to be a coloring agent.
Each of the printing heads produces on the foil a partial image corresponding to its coloring agent. The partial images produced by the printing heads are superimposed so that, after the last printing head, an overall color image is provided composed of the partial color images of the printing heads.
In transport direction of the foil after the printing heads, there is a fixing unit by means of which the images generated on the foil can be fixed. For forming a complete picture on the foil, the foil consequently, has to move along a predetermined path through the printing apparatus. The distance the foil needs to travel for generating a completely finished image corresponds to the length of the foil from the beginning of the first printing head to the end of the fixing unit.
Upon terminating the printing operation, it is therefore necessary that the last started image is moved through the printing apparatus until it has passed the fixing device. Since, at this point in time, there is below the printing heads an area of the foil to which no coloring agent was applied by the printing heads in the printing apparatus, after completion of the last image in the fixing unit, the foil needs to be moved back by the corresponding distance in a direction opposite to the travel direction. In this way, it is avoided that the foil has an imprinted area when operation of the printing apparatus is resumed.
Since the image generated on the foil consists of several partial images of different colors arranged on top of one another, it is necessary that the foil is moved through the printing apparatus with very high precision that is it needs to be moved past the printing heads in a highly precise manner. The smallest displacement of the foil would have the result that the individual partial images are not congruent on the foil whereby the contours of the final image would be blurred.
However, during the backward movement of the foil after completion of the last partial image, the foil is often slightly displaced, in particular sidewardly transverse to the direction of movement, which results in a position displacement of the foil. Only during printing operation, the foil re-assumes its correct position. That is the foil changes its position—starting from the incorrect position it assumed during shutdown of the apparatus—continuously up to the point where it is again in its correct position. The images generated during this period on the foil have therefore an inferior quality.
It is the object of the present invention to provide a printing apparatus of the type described above wherein however the quality loss of the images generated during start-up of the printing apparatus is very small or even negligible.